$ B = \left[\begin{array}{rr}-1 & 4 \\ 5 & 2\end{array}\right]$ $ A = \left[\begin{array}{rrr}1 & 2 & -1 \\ 1 & 4 & 1\end{array}\right]$ What is $ B A$ ?
Answer: Because $ B$ has dimensions $(2\times2)$ and $ A$ has dimensions $(2\times3)$ , the answer matrix will have dimensions $(2\times3)$ $ B A = \left[\begin{array}{rr}{-1} & {4} \\ {5} & {2}\end{array}\right] \left[\begin{array}{rrr}{1} & \color{#DF0030}{2} & \color{#9D38BD}{-1} \\ {1} & \color{#DF0030}{4} & \color{#9D38BD}{1}\end{array}\right] = \left[\begin{array}{rrr}? & ? & ? \\ ? & ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ B$ , with the corresponding elements in column $j$ of the second matrix, $ A$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ B$ with the first element in ${\text{column }1}$ of $ A$ , then multiply the second element in ${\text{row }1}$ of $ B$ with the second element in ${\text{column }1}$ of $ A$ , and so on. Add the products together. $ \left[\begin{array}{rrr}{-1}\cdot{1}+{4}\cdot{1} & ? & ? \\ ? & ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ B$ with the corresponding elements in ${\text{column }1}$ of $ A$ and add the products together. $ \left[\begin{array}{rrr}{-1}\cdot{1}+{4}\cdot{1} & ? & ? \\ {5}\cdot{1}+{2}\cdot{1} & ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ B$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ A$ and add the products together. $ \left[\begin{array}{rrr}{-1}\cdot{1}+{4}\cdot{1} & {-1}\cdot\color{#DF0030}{2}+{4}\cdot\color{#DF0030}{4} & ? \\ {5}\cdot{1}+{2}\cdot{1} & ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rrr}{-1}\cdot{1}+{4}\cdot{1} & {-1}\cdot\color{#DF0030}{2}+{4}\cdot\color{#DF0030}{4} & {-1}\cdot\color{#9D38BD}{-1}+{4}\cdot\color{#9D38BD}{1} \\ {5}\cdot{1}+{2}\cdot{1} & {5}\cdot\color{#DF0030}{2}+{2}\cdot\color{#DF0030}{4} & {5}\cdot\color{#9D38BD}{-1}+{2}\cdot\color{#9D38BD}{1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rrr}3 & 14 & 5 \\ 7 & 18 & -3\end{array}\right] $